A common method for improving the corrosion resistance of cold-rolled steel sheets, Zn-plated steel sheets and Zn-based alloy-plated steel sheets used for automobiles, household electronic appliances and building materials involves chromate treatment to form a chromate film. The chromate treatment is accomplished by the electrolyte chromate method or chromate application method. For the electrolyte chromate method, the bath used has been prepared, for example, by adding various anions to a mixture consisting mainly of chromic acid with addition of sulfuric acid, phosphoric acid, boric acid and a halogen, and treating the steel sheet by cathodic electrolysis. The chromate application method is associated with the problem of elution of chromium from the chromate-treated steel sheet, and therefore a method is adopted by which the hexavalent chromium is previously reduced with a reducing agent; for example, there are known sheets coated with an acidic aqueous solution composed mainly of trivalent chromium and containing a water-soluble chromate salt, an inorganic colloid compound and an inorganic anion, and sheets treated with a solution containing an inorganic colloid or inorganic anion with chromic acid of which part of the hexavalent chromium is reduced to trivalent, or chromic acid wherein the hexavalent chromium and trivalent chromium are in a specified ratio. There is also known a method of forming a composite with an organic polymer to immobilize the chromium and a method of further covering the chromate coating with an organic polymer.
Although those chromate coatings which are formed by electrolysis have low elution of chromium, their corrosion resistance cannot be said to be sufficient, while they have also had some problems in terms of scratch resistance of the coating during working and corrosion resistance after working.
Also, when chromate coatings formed by the application method are used in the form as applied, the chromate coating tends to undergo elution. Moreover, the corrosion resistance and coating adhesion have not always been sufficient, while the scratch resistance of the coating during working and corrosion resistance after working are also not always sufficient.
In the case of resin-type chromates, various types of resins are added to the chromic acid bath, at which time such resins gradually undergo reaction by the powerful oxidizing action of the chromic acid, making it difficult to maintain stability in the bath.
Thus, reduction of the hexavalent chromium beforehand in order to minimize chromium elution certainly reduces the corrosion resistance, while it does not completely prevent the chromium elution, and the adhesion is also insufficient. Also, in the chromium reduction method using a reducing organic polymer, the stability of the water-dispersable or water-soluble organic polymer itself becomes poor, while the chromium elution-preventing effect is low. The method of organic polymer coating also involves increased costs brought about by more steps, as well as problems such as chromium elution from damaged sections or cut sections of the coated film and impaired weldability, and for such reasons there have been proposed metal surface-treating compositions such as described in Japanese Unexamined Patent Publication No. 5-230666, which are composed of a mixture of an organic polymer aqueous emulsion of an organic polymer consisting of 0.1-10 wt % of an ethylenic unsaturated carboxylic acid component, 1-30 wt% of a hydroxyl group-containing monomer component and 60-98.9 wt % of another ethylenic unsaturated compound, stably dispersed in an aqueous medium, with a water-soluble chromate salt and an inorganic substance which reacts with an aqueous colloid and amphoteric metal to form a poorly water-soluble salt.
Also, in order to minimize interaction between the chromic acid and the resin, there has been proposed a technique whereby a fine powder of a poorly soluble chromate salt such as BaCrO.sub.4 or SrCrO.sub.4 is mixed in a resin solution, and a pigment is redispersed therein by mechanical means, for use as a rust-preventive coating (Japanese Unexamined Patent Publication No. 7-180068).
Japanese Unexamined Patent Publication No. 5-230666 mentioned above has a superior aspect in that a resin coating with excellent chromium elution properties is formed without having impaired corrosion resistance, but there are problems with long-term use of the resin coating, such as oxidation of the hydroxyl-containing component by the chromic acid and gelation by interaction between the chromic acid and carboxylic acid component. Also, the chromic acid elution-suppressing effect is believed to be caused by strong interaction of chromium ions with the resin, and when this interaction is too strong, uneven aggregation of the resin layer occurs (excessive interlocking between molecular chains, ion segregation, etc.), and the resulting coating is embrittled thus impairing the chromium elution properties at the worked sections; hence, it cannot be said that the chromium elution properties are adequate.
Furthermore, although the method disclosed in Japanese Unexamined Patent Publication No. 7-180068 involving redispersion of a powdered pigment is superior by rendering the chromium poorly soluble, despite the fine particle size of the primary particles of the pigment, there is a tendency toward aggregation of the pigment to occur during mixture with the resin solution. In addition, because of a tendency toward heteroaggregation with non-resins, the pigment dispersability in the treatment solution is poor, the treatment solution is destabilized due to precipitation of the pigment, and the pigment segregates in the film obtained by the coating, thus inevitably promoting corrosion at the sections with low pigment density. In addition, the pigment with increased particle size as a result of segregation in the coating when it is subjected to working breaks through the coating and exposes the underlying metal, and therefore the corrosion resistance of the worked sections cannot be ensured.
Another possible method is agitation of the solution with a homogenizer or mixer to produce powerful shear forces for redispersion of the aggregated and precipitated pigment in the treatment solution; however, this is not only uneconomical because of the increased number of steps but also results in eventual aggregation of the pigment in the solution, and therefore it essentially fails to provide a solution and does not guarantee stable dispersion throughout the coating.